In a typical radio communications system, user communications terminals referred to as user equipment units (UEs) communicate via a radio access network (RAN) with other networks like the Internet. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called a “NodeB” or enhanced Node B. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site.
Third Generation (3G) cellular radio systems like Universal Mobile Telecommunications System (UMTS) operating in Wideband Code Division Multiple Access (WCDMA) use different types of radio channels including unscheduled radio channels and scheduled radio channels. Mixed voice/data, circuit/packet switched 3G systems evolved from voice-centric, circuit-switched second generation (2G) systems. Unscheduled channels, sometimes called dedicated channels, are usually allocated to only one user for the duration of a connection carrying information only associated with that one user. Scheduled channels are packet-switched channels over which packets for multiple user connections are carried. Fourth generation (4G) systems, like the Long Term Evolution (LTE) of UMTS and Worldwide Interoperability for Microwave Access (WiMAX), design the air interface based on packet data. Dedicated traffic channels are eliminated in favor of scheduled radio channels in order to simplify the system. Medium access control is thus migrating towards a request resource-grant resource paradigm. In response to actual requests to transmit data from and/or to a user equipment (UE) in the uplink and/or the downlink, the scheduler in the base station dynamically allocates radio resources to satisfy the quality of service requirements associated with the type of data traffic to be transmitted, and at the same time, tries to optimize the system capacity.
The IEEE 802.16 Working Group on Broadband Wireless Access Standards develops formal specifications for the global deployment of broadband Wireless Metropolitan Area Networks (MAN). Although the 802.16 family of standards is officially called WirelessMAN, it is often referred to as WiMAX. In general, 802.16 standardizes two aspects of the air interface: the physical layer (PHY) and the Media Access Control layer (MAC). For the physical layer, one mode of IEEE 802.16e uses scalable orthogonal frequency division multiple access (OFDMA) to support channel bandwidths of between 1.25 MHz and 20 MHz with up to 2048 sub-carriers. IEEE 802.16e supports adaptive modulation and coding, so that in good radio signal conditions, a highly efficient 64 QAM coding scheme can be used, whereas in poor radio signal conditions, a more robust BPSK coding mechanism can be used. In intermediate conditions, 16 QAM and QPSK can be employed. Other physical layer features include support for multiple-in-multiple-out (MIMO) antennas in order to provide good NLOS (Non-line-of-sight) characteristics (or higher bandwidth) and Hybrid automatic repeat request (HARQ) for good error correction performance.
In terms of Media Access Control (MAC), IEEE 802.16e encompasses a number of convergence sublayers which describe how wireline technologies such as Ethernet, ATM and IP are encapsulated on the air interface, and how data is classified, etc. It also describes how secure communications are delivered, by using secure key exchange during authentication, and encryption during data transfer. Further features of the MAC layer include power saving mechanisms (using Sleep Mode and Idle Mode) and handover mechanisms.
The 802.16 WiMAX protocol supports five types of quality of service (QoS): UGS (Unsolicited grant service), rtPS (Real time polling Service), ertPS (Extended Real-time POLLING SERVICE), nrtPS (Non-real-time polling service), and BE (Best effort service). The Unsolicited Grant Service (UGS) is designed to support real-time service flows that generate fixed-size data packets on a periodic basis, such as T1/E1 and Voice-over-IP (VoIP) without silence suppression. UGS offers fixed-size, unsolicited radio resource grants (meaning the UE does not have to request a grant of radio resources before each transmission) on a real-time periodic basis, which eliminates the overhead and latency associated with UE grant requests and assures that grants are available to meet the data flow's real-time needs. Another term associated with UGS is semi-persistent scheduling of radio resources.
The unsolicited grants are allocated by the scheduler in the base station. UEs compete once for initial entry into the network, and thereafter, each assigned UE is allocated a UGS access slot by the base station scheduler. The granted UGS time slot can enlarge and contract, but it remains assigned to the UE for the duration of the UGS, which normally means that other UEs cannot use it.
But there are drawbacks with the UGS. The UGS consumes the radio resources in the same way as fixed, pre-assigned slots in TDMA systems such as GSM, regardless of whether the user is sending or receiving data over the UGS resource. For example, the UGS does not take advantage of the stochastic behavior, low data rate, and error tolerance of voice. Voice can be modeled as a stochastic process that has certain characteristics. Some vocoders generate null-rate frames during silence periods that can be omitted from transmission, therefore creating idle UGS slots over the air. Some vocoders generate full-, half-, quarter- and eighth-rate frames. Lower vocoder rates have lower power requirements and higher error rate tolerance, which makes bandwidth and power allocated in UGS slots more than what can be and actually is used. Other types of traffic may have similar characteristics, e.g., motion pictures with occasional still-image scenes.
One way to improve radio resource efficiency in facilitating services that use an UGS is to pack multiple users' low-rate packets (small payloads) into one big packet. This multi-user packet approach avoids allocating excessive bandwidth or power to a single low-rate user by aggregating small payloads into a large packet to fully utilize the available resources. But there are drawbacks with multi-user packets. First, multi-user packets are not used in the uplink. Second, extra constraints are imposed on scheduling when multi-user packets are transmitted on the downlink. The scheduler needs to maximize the usage of power and bandwidth and at the same time satisfy the delay constraint of each traffic flow, which introduces uncertainty in the formatting of multi-user packets. As a result of the format uncertainty, multi-user packets require more signaling overhead to indicate payload combinations, modulation, and coding schemes. Third, on the receiver side, blind detection may be necessary if payload combinations are compressed to reduce overhead. Compressed overhead adds extra hypotheses and hence degrades detection performance. Fourth, a multi-user packet complicates HARQ operations. Different user receivers may have different success in decoding a multi-user packet creating a dilemma in retransmission policy. If the same packet is retransmitted with less format signaling overhead, radio resources will be wasted on payloads that have been successfully received. If a different packet is retransmitted to avoid redundancy, the operations in the scheduler are more complex and more overhead for signaling the packet format is required.
Another approach that might improve radio resource efficiency in facilitating UGS communications is to use extended real-time polling where there is a period for silence suppression so that the uplink radio resources normally set aside for uplink transmission requests can be scheduled for data payload transmission. But extended real-time polling still requires uplink requests and downlink scheduling grants during the non-idle state where the traffic flow is nearly constant. Sending request and grant messages when the traffic flow is nearly constant, as it often is for voice traffic, is a waste of radio resources. During silence periods, some vocoders still transmit null frames to maintain the state machine at the receiver decoders. The null frames may be sent on the order of every hundred milliseconds, creating ambiguities for restarting polling. These low-rate null frames may keep the extended real-time polling largely in the “costly” request-grant operation. But not all vocoders support silence suppression. Some voice applications, such as SKYPE, generate continuous traffic. In those applications, the extended real-time polling reduces to real-time polling, where request and grant messages with the regular MAC overhead are constantly sent even though they are not necessary.
Consequently, an UGS envisioned for a request resource-grant resource-based packet radio communications system does not efficiently use radio resources for stochastic-type traffic that is non-deterministic in its activity, low-rate, low-delay, and error-tolerant.